Little to see: Fracking can exploit a wide area with few structures above ground
In early March, the media reported that protesters at a camp at Barton Moss near Manchester had won a legal battle to avoid eviction. They were there because the site was being used for exploratory drilling for shale gas – natural gas that is held underground in rocks with low permeability. To get the gas to flow freely, the geology has to be subjected to hydraulic fracturing or fracking, in which a mixture of water, sand and chemicals is pumped down the well shaft. The protesters claim that fracking causes earth tremors and pollutes drinking water aquifers.
These claims are discounted by John Blaymires, chief operating officer of IGas Energy – the company carrying out the drilling at Barton Moss. “Fracking is not the issue,” he says. “Safety is all about well design and construction.”
That was the message he had delivered just the month before at a symposium in London organised by the IMechE to examine shale gas extraction from an engineering perspective.
Blaymires, along with other speakers, believes that, when shale gas drilling and extraction are carried out in compliance with the regulations, they are as safe as any other form of hydrocarbon recovery.
More fundamentally, he argues that the case against shale gas extraction made by its opponents is simply wrong. For instance, he says that fracking has been carried out in more than two million wells worldwide without a single instance of contamination of drinking water traceable to the technique. Where the procedure has caused tremors, these have been of low intensity and associated with pre-existing geological conditions.
In a balanced energy generation scenario, he says, shale gas will tend to displace the use of coal and, as it has done in the US, can therefore contribute to reduced carbon dioxide emissions. And once drilling and fracking have been carried out, the surface installations required for shale gas extraction are minimal and do not ‘industrialise’ the landscape.
But in the UK, the situation has been bedevilled by the fact that the one instance in which fracking has been carried out did cause two small earth tremors. These occurred in 2011, following operations carried out by another company, Cuadrilla, at Preese Hall near Blackpool. An analysis commissioned by the Department of Energy and Climate Change, however, recommended that with safeguards – including the initial injection of smaller amounts of fluid and a lower threshold at which operations would be suspended if tremors were to be detected – there was no reason to suspend fracking at that site or elsewhere.
The contrast with the situation in the US is stark. There, exploitation of shale gas resources has grown to become a significant industry that could turn the country into a net exporter of liquid natural gas before this decade is out.
That reminder was delivered to the symposium by Chris Poole, head of engineering and technology development for Weir Group, which has been supplying wellhead equipment to the US industry for six years.
Artificially stimulating the flow of hydrocarbons from a well is not new, says Poole. The earliest attempts to do so in the US date back to the 1860s, and involved lowering explosive charges down the boreholes of oil wells. The fracking technique itself, though more recent, is also well-established in the US. Initial experiments took place in Kansas in 1947. Just two years later, the first commercial applications of the technique were carried out for oil exploration in Texas and Oklahoma by Halliburton.
Fracking for shale gas, however, did not take place in the US until the mid-1990s, and even then was initially inconsequential, he says. What made the US shale gas ‘revolution’ was its coupling with the technique of horizontal boring – making boreholes run out at angles of up to 90° from the initial vertical shaft – which was perfected during that decade.
A consequence of that combination of techniques, says Poole, has been pad-drilling – packing a series of vertical boreholes into a single, compact area at the surface, each borehole perhaps as little as 10m from its neighbours, and each also supporting horizontal extensions in different directions. That way, a wide underground volume can be exploited with little surface structure.
Once the boreholes have been drilled, the subsequent fracking involves no permanent structures but only portable, or easily removable, equipment, he says. Typically, this process will involve tanks for the water and chemicals, with all the other equipment being truck-mounted.
The US shale gas industry is continuing to develop in ways that pose challenges for companies such as Weir, says Poole. He identifies several such trends.
One trend derives from a change in the fracking technique. A single well is fracked at several points along its horizontal bore, he says. In the earliest days of the industry, the technique involved creating a series of fractures along its length, starting at its most distant point with the bore, then sealed through the insertion of a plug just in front of the fracture before another fracturing exercise was carried out further back towards the vertical bore.
Once all the fractures were complete, the plugs would be drilled out and the gas allowed to escape to the vertical bore and then to the surface. Using this technique, the pumping operations required to insert the fluid were intermittent and lasted only the few hours necessary to complete a single fracture before its accompanying plug was created.
Now, though, this ‘plug and play’ technique is being supplanted by a ‘sliding sleeve’ approach which enables all the fractures to be carried out in a single continuous operation that may last 40 or more hours. This method reduces the overall time from the first fracture to the last, but the requirement for continuous operation imposes greater demands on the equipment involved. The specifications for a typical fracking pump might now require pumping pressures in the range 10,000-22,000psi (68,948-151,685kPa), power ratings of 2,000-3,000hp (1,491-2,237kW) and a maximum rated speed of 300rpm.
Another refinement in fracking – one that makes the technique more environmentally friendly – is making life tougher for the equipment involved. This is a trend to reuse ‘returned water’ – that has already been used in a fracking operation – without treatment to remove the contaminants, mainly salt and particles of rock, that it will have picked up during that application.
In the early days, there was an assumption that the water used in a fracturing operation had to be of the same quality as that delivered from a domestic tap, says Poole. Hence the water recovered from a fracturing operation – about 80% of that initially injected – had to be taken off-site for the appropriate treatment.
Now, though, returned water is increasingly being reused. This approach was pioneered by Halliburton, the company that carried out the first commercial fracking operations. It means the fluid that passes through the pumps will be both “corrosive and erosive”, says Poole. Weir has enhanced its equipment to allow it to cope with this new set of demands.
If the US experience were to be repeated on this side of the Atlantic, just how much shale gas is there underneath the UK’s landmass?
Analysis of the available data is likely to come up with only an approximation, says Mike Stephenson, director of science and technology for the British Geological Survey (BGS), part of the UK’s Natural Environment Research Council, which last year published probably the most detailed attempt to estimate shale gas resources in a particular area yet carried out in Europe.
The report, commissioned from the BGS by the Department of Energy and Climate Change, focused on one of the UK’s primary areas for prospective shale gas exploitation – the Bowland-Hodder shale formation that lies under an area encompassing much of Lancashire and Yorkshire and, at its southern fringe, parts of Cheshire, Derbyshire, Nottinghamshire and Lincolnshire.
The study is based on information derived from seismic and geological testing in the past and on core samples extracted from the boreholes that have been drilled in the area previously, says Stephenson. The resulting data was then loaded into a 3D computer model to produce what he says is an accurate representation of shale formations in the area.
Nevertheless, the study provides only a broad estimate of the region’s potential. The reason for this involves an understanding of the distinction between the terms ‘resources’ and ‘reserves’, he says. The first refers to the total shale gas that may exist in an area, whereas the second is the proportion of that amount that may be practicably recoverable.
The amount of reserves depends on geology and technology but also political and social aspects. Whatever the amount of shale gas that may exist, public opinion must accept that it can be extracted safely as well as economically.
As for resources, not only is it necessary to know the extent of the shales involved but also the actual concentration of hydrocarbons they may contain. Ultimately, the only way to discover that is to drill into them, says Stephenson. So until targeted exploratory drilling takes place, only estimates based on likely concentrations can be hazarded.
Nevertheless, even this one area could offer substantial prospective resources – the report estimates that at least 23.28 trillion m3 of shale gas are likely to be in place and possibly as much as 64.62 trillion m3.
To put those figures in context, UK imports of natural gas in the first six months of 2013 were around 28.3168 billion m3.
The Bowland region is just one of several areas in the UK that include shale formations likely to contain recoverable gas. Others are the central Lowlands of Scotland, Hampshire, South Wales and the Weald area in southern England. In the last-mentioned area, the BGS has been carrying out a similar exercise to estimate resources, and the resulting report is due for publication soon.
The UK is coming to a pivotal decision-making point in its energy policies. A potentially massive resource exists that can be exploited without the need for any great technological innovation. Instead, the battle will most likely involve fears of environmental degradation and prejudice against the use of hydrocarbon fuels.